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Officials: Too soon to disregard 9/11 threat

FBI director: "It is an outstanding threat that we are following up on"

Official: More than 300 interviews have been conducted

It may be impossible to tell if a plot was thwarted or whether none existed

Washington (CNN) -- Top U.S. officials say they are not ready to put aside threat information received last week that al Qaeda terrorists wanted to attack New York or Washington around the time of the 10th anniversary of the September 11, 2001, attacks.

"The threat has not been resolved, and until it is resolved it is an outstanding threat that we are following up on," said FBI Director Robert Mueller. "Even though September 11th has now passed we do not believe that necessarily means that we should back down."

Last Thursday, the Department of Homeland Security confirmed the U.S. had specific and credible intelligence a plot could be in motion, but officials stressed the information was uncorroborated. That information led New York, Washington and other cities around the country to ramp up already-tight security for the anniversary of the attacks, which killed nearly 3,000 people.

The FBI has conducted more than 300 interviews, and cleared all of those people of being involved in or knowing about a possible plot to attack the cities with vehicle-borne explosives or some other form of violence, a federal law enforcement official said Monday.

Mueller told the Senate Homeland Security and Governmental Affairs Committee numerous interviews helped agents eliminate some avenues in the investigation but "there is still work to be done."

U.S. officials say they have no proof al Qaeda operatives entered the United States but are continuing to follow leads.

Sen. Joseph Lieberman asked on Tuesday who was in charge of coordinating the response to the threat information and officials pointed to John Brennan, the assistant to the president for Homeland Security and Counterterrorism.

Matthew Olsen, the director of the National Counterterrorism Center, said his organization brought together all the information coming from the CIA, FBI, Homeland Security and other agencies.

"We play a central clearinghouse role where we take all of the information, analyze it and share back out what we are seeing from an analytical standpoint," said Olsen.

It may not be possible to determine whether tough law enforcement measures prevented a plot from being carried out -- or if one had never been in the works, two U.S. officials, who were not authorized to speak on the record, said Tuesday.